Salmonella are facultative intracellular pathogens which cause significant diseases in humans and animals. These organisms are responsible for several disease syndromes, including enteric (typhoid) fever, gastroenteritis, bacteremias and focal infections. Typhoid fever is a severe systemic illness which is mostly a problem in the developing world and in travelers. Non-typhoidal salmonella infections are increasing in the USA and are largely associated with contaminated food. Salmonellae infections are most severe in infants, the elderly, and in immunosuppressed individuals. This grant proposes to study the mechanism by which Salmonellae survive host innate immune killing. Innate immune killing involves the non-antigen specific mechanisms by which animals eliminate invading bacteria. Included in innate immune mechanisms are antimicrobial peptides produced at mucosal surfaces and within phagocytic cell granules and cytokines produced in response to recognition of bacterial lipid A. Pathogens such as Salmonellae have mechanisms to resist these killing mechanisms that are environmentally regulated. The genes encoding these mechanisms are the subject of this grant. They include the virulence regulators PhoP/PhoQ that respond to signals within host tissues and induce genes necessary for resistance to innate immune killing. These regulators are essential for human and animal virulence. PhoP/PhoQ regulate genes involved in surface remodeling of bacteria. These genes include those responsible for modification of the lipid and protein components of the outer membrane. This grant proposes to define the mechanism by which these modifications are generated and the role of surface remodeling in bacterial virulence. The specific aims of this proposal are to define the genes involved in lipid A modification and the effects of these modifications on bacterial virulence and host cell recognition of lipid A. In addition a variety of genomic and proteomic techniques will be used to fully define the genes regulated by PhoP/PhoQ to better understand the coordinately regulated response of bacteria to host colonization.